A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is desirable to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
The pattern is transferred onto several successive resist layers on the substrate in order to build up a multi-layer structure with the pattern throughout its thickness. It is therefore desirable to ensure that the pattern in any given layer is exactly aligned with the pattern in the previous layer. The way that successive patterned layers are aligned is by having overlay targets in the layer, these overlay targets being detectable by an inspection beam that is projected by the projection system before the exposure beam is projected to apply the pattern. In order to leave as much space as possible on the substrate for the exposed pattern, the overlay targets are positioned in scribe lanes, which is the part of the substrate that will be sawn to separate the substrate into individual ICs, for example. Overlay targets have, in the past, taken the form of stacked (in several or all the layers) copper areas alternating with dielectric areas. Overlay targets may also be used for alignment of a substrate with respect to a substrate table or other fixed object.
As lithographic techniques improve and smaller patterns are possible, smaller ICs are also possible and so the area between the scribe lanes decreases. If the scribe lanes stay the same size while the “usable” area between them gets smaller, the ratio of unusable substrate to usable substrate increases, reducing efficiency of the substrate use. The present use of relatively large copper areas in the overlay targets in the scribe lanes means that the size of the scribe lanes is difficult to decrease and so inefficient use of substrate space is inevitable. Overlay targets are typically in the form of gratings made up of parallel bars. The pitch of the grating should be a similar order of magnitude to the product that is eventually to be manufactured on the substrate so that overlay to the correct accuracy can be measured. Presently, the minimum pitch available is around 400 nm. However, modern designs generally require a pitch of 300 nm or smaller. For this to be workable, the wavelength of the inspection beam used to irradiate the target would need to be less than 450 nm. However, tuning tolerance with a beam this high in frequency would be very limited.